A secondary cell can exhibit a thermal runaway due to various causes such as an internal short-circuit and an overcharge. For example, when a lithium-ion secondary cell exhibits a thermal runaway, the cell temperature can rapidly increase to 300° C. to 400° C. or more. When any secondary cell exhibits a thermal runaway and causes a thermal runaway of its adjacent secondary cell, disadvantageously, many secondary cells exhibit a thermal runaway and the energy of the thermal runaway extremely increases. This disadvantage can be eliminated by burying all or some of the cylindrical cells in a thermally conductive resin and by absorbing, into the thermally conductive resin, the thermal energy of the cylindrical cell having exhibited the thermal runaway (Patent Literature 1 and Patent Literature 2).